100207-0841 EST
Assuming you have a good EGC at the lamp load point, then measure the neutral to EGC voltage at this point. If it fluctuates by your 10 V difference or thereabouts, then it is at least in part a neutral problem. Under nominal load conditions on a circuit in my house I see a volt or so between neutral and EGC, and not fluctuating if the load on that circuit is not changing.
If the neutral to EGC voltage has little fluctuation, then monitor the hot to EGC voltage.
If you do not have a good EGC, then use a long wire (an extension cord can be used) from the ground-neutral point in the main panel as an extended test lead to your test point. A three conductor extension cord is very convenient to pickup the ECG from the main panel if there is a mating outlet near the main panel.
Assume you have a circuit designed for a maximum voltage drop at maximum circuit load from the main panel to the load of 3%, then about 1.5% of this drop should be on the neutral. At 120 V that would be 1.8 V.
Your strobing effect may not result from the voltage fluctuations. On a magnetic ballast 8' Slimline I can drop to about 75 V before there is a light flicker problem.
.
